With the development of Integrated Circuit (IC) manufacturing processes, new packaging methods have been invented and applied to a variety of IC products. For example, Fine-Pitch Ball Grid Array (FPBGA) technology is used for most logic ICs while Thin Small Outline Package (TSOP) remains the main stream of memory packages. To choose a proper packaging type for a IC product, a designer should take the application into consideration, i.e. numbers of I/O signals, frequency, operation temperature, space, cost, etc.
However, popular packaging methods may not apply to special cases that may require particular geometry or physical requirement. For instance, a photo sensor or a capacitive fingerprint sensor IC needs to have the sensing area exposed for picking up light or electrical energy. Therefore, a commonly used packaging type would have a sealing material over only the non-sensing area, including the wire-bonded pads, and leave the sensing area exposed to the environment as shown in FIG. 1. FIG. 1 shows a cross-sectional structure of a fingerprint sensor package. The fingerprint sensor package mainly includes a substrate 11, a fingerprint sensing chip 12, a number of bonding wires 13 and a sealing material 14. An active surface 21 of the fingerprint sensing chip 12 has a sensing region 23 formed thereon. The back of the fingerprint sensing chip 12 is adhered to a top surface 11a of the substrate 11. The fingerprint sensing chip 12 is electrically connected to the substrate 11 with the bonding wires 13. The sealing material 14 is formed on the top surface 11a of the substrate 11 to seal the bonding wires 13 and the non-sensing portion of the fingerprint sensing chip 12. The sensing region 23 must be exposed. There are several bumps 15 formed on a bottom surface 11b of the substrate 11 as signal connectors to receive power and to electrically communicate with external circuitry.
It is clear from FIG. 1 that a protrusion height H of sealing material 14, including a height h of bonding wires 13 and that of bonding material for covering the wires, cannot be avoided. When portable devices, such as a smart phone or smart card, which carry the fingerprint sensing chip 12 become more compact and slim than ever, the protrusion height H is preferred to be eliminated so that the surface of portable devices can be perfectly flat.
Flip Chip assembly is another popular packaging method whose package size is smaller than a traditional carrier-based package. However, for a contact image sensor or a capacitive fingerprint sensor IC which may require both a compact package and a flat contact surface, flip chip assembly may not suitable. A typical flip chip assembly is shown in FIG. 2. Attachment pads 2 of a chip 1 and contact pads 5 on a PCB 4 are electrically connected using solder balls 3, and an electrically-insulating adhesive 6 is then underfilled to provide a stronger mechanical connection. A surface level difference H′ between the upper surface of the chip 1 and the upper surface of the PCB 4, which is the sum of the height of the chip h1 and that of the solder balls h2, makes a flat top surface of the package impossible.
Besides the technologies mentioned above, another technology involves chip-scale packaging is disclosed by U.S. Pat. No. 8,736,080. U.S. Pat. No. 8,736,080 of ARNOLD et al. discloses a compact Integrated Circuit Assembly comprising: an integrated circuit, a substrate where the integrated circuit is disposed on, a conductive layer disposed in the signal trench and coupling to the integrated circuit signal pad, a bond wire configured to couple the conductive layer to an external pad. The substrate comprise at least one signal trench which proximate to an integrated circuit signal pad and extending to one edge of the substrate. The bond wire, the at least one signal trench and the conductive layer are formed below a surface plane of the integrated circuit. This method successfully reduces the height of the package, and also provides a flat top surface. However, the manufacture process, involving a deep etching step to form the trench and an additional metal plating step to form the conductive layer, requires more manufacture time and additional cost.
Therefore, in order to solve the problem mentioned above, a chip with pads on the peripheries and a method to manufacture the chip is provided. The chip can be a fingerprint sensing chip, even a contact image sensor.